Assembly and Test of the Gas Pixel Detector for X-ray Polarimetry

TL;DR

This paper presents the design, assembly, and preliminary testing of a gas pixel detector for X-ray polarimetry, demonstrating promising energy resolution, modulation factor, and systematic error control suitable for space missions.

Contribution

It reports the first in-house assembly and testing of a GPD with optimized gas mixture, achieving uniform electric field and key performance metrics for X-ray polarimetry.

Findings

Energy resolution of 18% at 6 keV

Modulation factor up to 0.6 above 6 keV

Residual modulation less than 0.3% at 6 keV

Abstract

The gas pixel detector (GPD) dedicated for photoelectric X-ray polarimetry is selected as the focal plane detector for the ESA medium-class mission concept X-ray Imaging and Polarimetry Explorer (XIPE). Here we show the design, assembly, and preliminary test results of a small GPD for the purpose of gas mixture optimization needed for the phase A study of XIPE. The detector is assembled in house at Tsinghua University following a design by the INFN-Pisa group. The improved detector design results in a good uniformity for the electric field. Filled with pure dimethyl ether (DME) at 0.8 atm, the measured energy resolution is 18% at 6 keV and inversely scales with the square root of the X-ray energy. The measured modulation factor is well consistent with that from simulation, up to ~0.6 above 6 keV. The residual modulation is found to be 0.30% +/- 0.15% at 6 keV for the whole sensitive area, which can be translated into a systematic error of less than 1% for polarization measurement at a confidence level of 99%. The position resolution of the detector is about 80 um in FWHM, consistent with previous studies and sufficient for XIPE requirements.